1. Field of the Invention
The present invention relates to a sealant layer applied between a positive electrode case and a sealing body of an alkaline dry battery.
2. Description of the Prior Art
A positive electrode case of an alkaline dry battery is a sealed case obtained by sealing an opening portion thereof with a terminal plate serving as a negative electrode terminal via an insulating sealing body. Conventionally, in order to improve the leakage proof property in such an alkaline dry battery, there have been configurations in which a sealant layer is provided between a positive electrode case and a sealing body.
An insulating material with a high viscosity is normally used for a sealant layer. However, there still have been studies on materials which can prevent degradation of the leakage proof property under various use environments.
For example, a material (such as a mixture of polyisobutylene, polybutene and polyolefin crystalline resin) having a high viscosity even in a high temperature range is described as a sealant layer of which a leakage proof property is not degraded with respect to a temperature cycle in Japanese Laid-Open Publication No. 5-290820.
Moreover, there are cases where a material obtained by adding nickel oxyhydroxide to manganese dioxide is used as a positive electrode mixture in order to improve the current discharge property. However, since nickel oxyhydroxide is highly oxidative, the sealant layer provided between the positive electrode case to which a positive potential is applied and the sealing body is easily degraded through oxidation. As a sealant layer suitable in such a case, a material (such as a mixture of blown asphalt and polybutene) having an excellent oxidation proof property is described in Japanese Laid-Open Publication No. 2005-353308.
There are also cases where a sealant layer is provided between a negative electrode current collector connected to a terminal plate and a sealing body in order to improve a leakage proof property of an alkaline dry battery. In such a case, if a material used as a sealant layer has a water absorbing property, a sufficient leakage proof property might not be able to be achieved under a humid condition. A possible reason for this is as follows. If moisture exists in a sealing portion between a negative electrode current collector to which a negative potential is applied and a sealing body, a reaction expressed by H2O+e−→OH−+½H2 occurs in the sealing portion, so that cations such as K+ ions in an alkaline electrolyte move and a creep phenomenon is easily caused. As a sealant layer suitable for such a case, a material (such as a mixture of polyamide resin and synthesized oil) having an excellent repellency is described in Japanese Laid-Open Publication No. 2007-26924.
As another sealant layer, for example, a material (such as a mixture of epoxy resin and polyamide resin) having a high sealing property at a high temperature is described in Japanese Laid-Open Publication No. 57-065668 and a material (such as polyamide resin of which a softening point is within a temperature range from 40° C. to 60° C.) with a high ductility is described in Japanese Laid-Open Publication No. 62-126545.